This invention relates to adjusting the displacement of a fluid pump. In particular, the invention relates to adjusting the starting load on an engine-pump that supplies pressurized hydraulic or pneumatic fluid for driving the wheels of a vehicle having a hybrid powertrain.
A hybrid powertrain motor vehicle may include various sources of power including an internal combustion engine, which drives a fluid pump, and other on-board sources of fluid pressure, such as an accumulator. Pressurized fluid is supplied to hydraulic or pneumatic motors, which drive the vehicle wheels. Generally, such a hybrid powertrain includes a power accumulator containing fluid at relatively high pressure and a regeneration accumulator, in which kinetic energy of the vehicle, recovered from a brake regeneration system, is stored in the form of pressurized fluid. The accumulators and pump supply fluid to the motors at the wheels through a high pressure rail. Fluid exiting the fluid motors is returned to a reservoir, from which fluid is drawn to the pump inlet.
The stroke of the fixed displacement pump driven by the engine is a constant. The magnitude of pressure in the supply rail varies according to the degree to which the driver demands output power, the frequency and magnitude of brake energy recovery events, the energy storage capacity of the accumulators, and other unpredictable factors including road conditions. When the engine is turned off, the magnitude of supply rail pressure is influenced by these conditions. Upon restarting the engine, the starting load on the engine and pump is affected by the magnitude of supply rail pressure.
Because there is little control over supply rail pressure and no control over the stroke of the fixed displacement pump, the engine may be required to start repeatedly against a large load, the pressure in the supply rail. Certain engines, such as a free piston engine or a conventional internal combustion engine operating with homogeneous combustion compression ignition, perform best when the amount of fuel supplied to the engine, the engine compression ratio, and the air-fuel ratio are controlled for each engine cycle within a close tolerance, even at engine startup. If these parameters are not maintained within narrow tolerances for each engine cycle, such engines are susceptible to starting difficulties and stalling.
To avoid these difficulties, it is preferred that such engines be started with idle fuel quantities so that the engine can respond to a demand for maximum power output after a large number of engine cycles have occurred after starting, rather than immediately upon startup. To accomplish this desired reduction in starting load, even when supply rail pressure is high, a technique is required to reduce the effective load on the engine for a period during and immediately after engine startup.